1. Field of the Invention
This invention relates to an electric heating pad, and, more particularly, to a flexible electric heating pad having a self-limiting heating element and method of making such heating pad.
2. Description of the Prior Art
Electric heating pads are widely used for therapeutic and general comfort purposes. Heating pads currently available generally utilize fixed electrical resistance type heating elements. A problem that arises with fixed resistance type heating elements is that they are subject to overheat conditions if excess heat is not dissipated from the heating element. Overheat conditions often arise when the user of the heating pad places his or her body over the heating pad, as by laying or sitting on the pad. This often occurs when the heating pad is used in bed. When used in this manner, all of the surface area of the pad is covered and excess heat cannot dissipate from the heating element. Overheat conditions can also arise when clothing or other covering material is placed over the pad. Thermostats may be used to partially remedy the problem of overheat, but thermostats will not work if the area of overheat is located some distance from the thermostat.
Because of the possibility of overheating occurring, fixed resistance heating pads must be placed over the area where the application of heat is desired with the opposed pad surface being exposed to the air. This may require that the user position himself or herself in an uncomfortable or inconvenient manner.
When overheat conditions occur, the temperatures attained often result in breakdown or combustion of the insulation material on the heating element or combustion of the heating pad covering material. As a result, there is serious risk of fire and injury when overheat conditions occur in such heating pads.
Another problem with fixed resistance heating pads is that the pads lack flexibility. That is, if the pad is flexed, or rolled up, it may damage the electrical integrity of the heating element.
The use of positive temperature coefficient (PTC) material to provide self-limiting heating elements has been known. U.S. Pat. Nos. 4,271,350; 4,309,596; 4,348,584; 4,818,439; and 4,277,673 disclose examples of heating elements which include various types of PTC materials. PTC material exhibits variable electrical resistance with temperature. As the temperature of the material increases, its electrical resistance also increases. When the material reaches a predetermined maximum temperature, its electrical resistance is effectively infinite and, the material will not heat past that predetermined maximum temperature. The predetermined maximum temperature is determined by the composition of the PTC material. PTC material typically includes a polymer in which electrically conductive particles are suspended. A typical example is disclosed in U.S. Pat. No. 4,277,673.
Because of the characteristics exhibited by PTC materials, heated articles using PTC containing heating elements are much less susceptible to overheat conditions. U.S. Pat. No. 5,081,339 discloses a waterbed heater which utilizes a PTC heating element. U.S. Pat. Nos. 4,309,596 and 4,348,584 disclose examples of PTC heating elements which it said may be used in electric blankets.
Some problems associated with PTC heating elements include decreased flexibility of the heating elements as the size of the heating element is increased and decreased stability of the PTC material with aging. In addition, small PTC heating elements which produce sufficiently high temperatures for use in the heating pads are often not sufficiently flexible for use in such articles.
PTC heating elements may present fire hazards if short or open circuit conditions occur therein. It has been known to utilize safety circuits which non-resettably discontinue electrical power from the heating element should short or open circuit conditions occur. An example of such a circuit is disclosed in U.S. Pat. No. 4,436,986.
Despite the prior art systems, there remains a need for a flexible electric heating pad which minimizes risk of overheating conditions occurring. There also remains a need for a PTC heating element that provides improved heating performance, which provides increased flexibility of the heating element and which provide improved stability of the PTC material.